The present invention relates to producing electricity on board vehicles and containers, for example.
It is known to produce electricity on board vehicles or containers by means of devices comprising an alternator driven by an engine, a rectifier, and a voltage-raising chopper.
A large number of such on-board electricity-producing devices have been proposed, in particular with the purpose of reducing the weight of the equipment.
Thus, numerous devices have been proposed in which the voltage-raising chopper uses the self-inductance of the alternator, thus making it possible to avoid using a coil specific to the chopper.
In particular, application GB-A-2 289 581 discloses a device comprising a conventional three-phase alternator, a rectifier, and a half-bridge chopper downstream from the rectifier.
U.S. Pat. Nos. 5,793,625 and 4,825,139 describe other devices in which the chopper uses the self-inductance of the alternators.
The installations that are powered electrically by means of the electricity-producing device that is present on board a vehicle or a container are, for example, refrigerator installations.
When the engine is stopped, these installations must be able to continue operating.
The invention seeks to satisfy this need.
The present invention thus provides a novel device for producing electricity, and comprising:
a three-phase alternator having no motor function, comprising three windings, said alternator having self-inductance;
a rectifier for converting the alternating current (AC) delivered by the alternator into rectified current;
a voltage-raising chopper having at least one switch element enabling a higher voltage to be generated across the terminals of the alternator by means of the self-inductance of the alternator; and
a switch unit enabling the three windings to be connected either in a star configuration when the alternator is being rotated to produce electricity, or else to connect the three windings to respective phases of a three-phase network when the alternator is stopped.
The Applicant company has found that the coils of the alternator can be connected directly to a three phase network in order to power the rectifier.
There is therefore no need to provide a rectifier other than the rectifier provided for the alternator, thus providing savings in weight and cost, and increases in reliability and simplicity.
In addition, by having the chopper present downstream from the alternator it is possible to deliver variable power to the load circuit(s), and thus cause them to operate under conditions that are best suited to circumstances.
In particular, the device can power a regulator serving to vary both the frequency f and the voltage U while keeping the ratio U/f constant, for example.
This makes it possible for example, to cause a constant torque motor to operate at variable power regardless of the frequency and the voltage of the power supply network, and without any increase in mass or volume.
A motor delivering power of 5.5 kilowatts (kW) at 400 volts (V) and at 50 hertz (Hz) can have its power raised to 6.6 kW at 460 V and 60 Hz and to 7.5 kW at 500 V and 70 Hz.
In addition, by connecting load circuits via the rectifier and the windings of the alternator, in particular, it is possible to improve the power factor.
It is preferable to use an alternator having eight poles and for the switching element to operate at a switching frequency lying in the range 3 kilohertz (kHz) to 15 kHz, while preferably remaining below 8 kHz and more preferably equal to 7.5 kHz.
The Applicant company has found that it is possible to make the device of the invention operate at such a switching frequency with satisfactory overall efficiency.
The self-inductance in percentage terms per phase of the alternator is defined by the formula Lxcfx89IN/V0, where L is the self-inductance of the alternator, xcfx89 is the angular frequency, IN is the nominal current at a given speed, e.g. 1500 revolutions per minute (rpm), and V0 is the unloaded voltage of the alternator at said speed, and it preferably lies in the range 15% to 40%, and more preferably in the range 20% to 30%.
In a preferred embodiment corresponding to power of about 10 kW, with the voltage being 294 volts between phases at 3000 rpm, the inductance per phase of the alternator is less than or equal to 2 millihenrys (mH), preferably less than equal to 1.5 mH, which corresponds to inductance in percentage terms lower than 30%.
Still in an embodiment corresponding to power of about 10 kW, for a voltage of 294 volts between phases at 3000 rpm, the inductance per phase of the alternator is greater than or equal to 1 mH, which corresponds to inductance in percentage terms greater than 20%.
The Applicant company has found that these values of inductance and frequency make it possible with an eight-pole alternator to keep the losses due to current harmonics to a level that is particularly low.
The alternator is preferably a permanent magnet alternator.
Also preferably, the chopper is downstream from the rectifier, thus making it possible to use a single transistor, for example an insulated gate bipolar transistor (IGBT).
This single transistor can be cooled easily by circulating a liquid coolant, whereas in conventional devices where the choppers have a plurality of transistors, cooling is made more difficult by the number of transistors, since they cannot be mounted on a common cooler without giving rise to problems of insulation.
The switching frequency is preferably constant, thereby simplifying the control electronics.
The chopper can be connected directly to the rectifier without a coil between them.
In a preferred embodiment, the chopper feeds at least one inverter controlled by a control circuit receiving information representative of the speed of rotation of the rotor of the alternator.
The control circuit can thus take account of the power available for the loads connected to the inverter, and can control the operation thereof accordingly.
By way of example, the device makes it possible to provide electrical power to a refrigerator system having at least one motor.